In this project, lead by Peter Berg from the University of Virginia at Charlottesville , we develop and test the Eddy Correlation Method, a new technique that permits non-invasive oxygen flux measurements at the seafloor. “Non-invasive” in this context means that this technology does not interfere with the flow and light regime at the study site, in contrast to benthic flux chambers that exclude the natural bottom layer currents. The main goals of this project are to further develop the technique, enhance the accessibility of the Eddy Correlation Instrument (Fig. 1) to users, and assess the potential of this technique for long-term deployments in monitoring programs. For more information, please visit http://faculty.virginia.edu/berg/

Fig. 1 Left: The sensor head of the Acoustic Doppler Velocimeter and the tip of the oxygen electrode of the Eddy Correlation Measurments. Right: Eddy Correlation instruments deployed in an estuarine environment in the Northern Gulf of Mexico.

The exchange of O2 between the seafloor and the overlying water is a key parameter in studies of marine systems as it is a good proxy for the sedimentary production and decomposition processes (Canfield et al. 1993) . However, the oxygen flux is tightly linked to the characteristics of the boundary layer flow, thus, any interference with this flow environment entails changes in the oxygen flux. For example, benthic chambers exclude oscillating flows as produced by waves and tidal currents, and this oscillation can have strong influences on the oxygen flux (see project “Biocatalytical filtration and carbon cycling in permeable shelf sediments”). Likewise, the temporal dynamics of the flux are coupled to the flow environment, thus, a method not interfering with the bottom currents is required to resolve the effect of the variations in magnitude and direction of flow.

The Eddy Correlation Technique for aquatic environments is an adaptation of the Eddy Correlation method developed for atmospheric boundary layer research, where Eddy Correlation now is the standard approach to measure land-air exchanges. When used in aquatic environments, the technique derives oxygen flux from synoptic, high frequency records of vertical water motion and oxygen concentration measured simultaneously in a small measuring volume above the sediment. The flux calculation is based on the assumption that all O2 transported vertically towards or away from the sediment surface is facilitated by turbulent water motions. Oxygen consumption in the sediment, thus, causes that upward directed turbulent motion would carry water with low oxygen concentration through the measuring volume, while downward directed flow would move water with higher oxygen concentration through that volume (Fig. 2)

Fig 2. The fluctuating O2 concentration (upper pane) and the fluctuating vertical velocity (lower pane) measured at the same point 15 cm above a sediment surface. Positive velocities indicate a flow up and away from the sediment. The thick black lines depict the smoothed O2 concentration and the smoothed vertical velocity. (Adapted from Berg et al. (2003).

The averaged O 2 flux is calculated as

where the bars symbolize the averaging over the time series, and uz' and O2' are the turbulent fluctuating components of the vertical velocity and the O2 concentration. Magnitude and direction of flow and oxygen concentrations are measured using an Acoustic Doppler Velocimeter and a fast O2 microsensor.

The sediment surface area that contributes to the flux recorded by the Eddy Correlation instrument is called the “Footprint Area” and is located upstream from the measuring point (Fig. 3). It has an elliptic shape and its size depends on the measuring heights above the sediment and bottom roughness. In very shallow water, oxygen flux through the water-air interface also controls size and shape of the footprint.

Fig. 2. (A) Typical footprint seen from above. The black dot marks the measuring point. In this example, the boundary of the footprint delineates the area that contributes 90% of the measured flux, the rest comes from the surrounding area. The two lower panes show the normalized flux contribution along the X (B) and Y axis (C). (Adapted from Berg et al. 2007)

Figure 3 shows that the measuring point is located at the edge of the sediment surface that causes the flux. Light and flow field over the area contributing to the flux therefore are not affected by the instrument permitting for the first time measuring O2 exchange between seafloor and overlying water under natural flow conditions.

It is not possible to fully explain O2 exchange over the sediment-water interface of shallow water sediments without taking benthic primary production into account. Light intensities reaching the bottom often are sufficient to drive photosynthesis to depths of more than 40 m (Nelson et al. 1999) modulating net O2 fluxes significantly. Ensuing complex changes in sediment-water O2 flux are further complicated by the fluctuations in boundary flow and changes in bottom topography altering turbulence spectra and advective pore water exchange. We use the advantage of the non-invasive flux measurement technique to determine the influences of the natural bottom currents and light variations on the O2 exchange.

In this project, we explore the advantages of the eddy correlation technique relative to traditional methods and investigate the suitability of different sensors for measurements of other dissolved species. The development seen in the atmospheric sciences, where land-air exchanges of various quantities are now measured almost exclusively by eddy correlation, suggest that this technique could become a standard method for flux measurements in the aquatic environment.

Instrument development

We demonstrated that fast optical oxygen sensors (Fig. 4), known as optodes, represent a good
alternative to the traditional Clark-type electrochemical microelectrodes for such measurements. Optodes have
the advantage over microelectrodes of being insensitive to flow, less susceptible to signal drift, more durable
under typical field conditions, less expensive, and repairable. Comparisons of the response times of optodes and
microelectrodes to rapid oxygen changes showed that optimized optodes had the same response time (162 ± 66
ms) as the microelectrodes (160 ± 57 ms) and were fast enough to capture the oxygen fluctuations that are relevant
for the eddy correlation flux calculations. Side by side comparisons of benthic O2 flux data collected with
microelectrode-based eddy correlation instruments and optode-based eddy correlation instruments in freshwater
and marine environments showed no significant differences between the measured fluxes. The optodes
allow the development of more user-friendly eddy correlation instruments that combine the advantages of noninvasive
measurements and integration of fluxes over a large footprint area, using a relatively rugged and less
expensive sensor.

Fig.4. (A) Eddy correlation instrument with oxygen optode. (B) ADV sensor
head and two optodes. (C) Optodes housed in 1 mL syringe. The dark
spot at the end of the glass fiber protruding from the needle is the sensing
dye. The dark line parallel to fiber is the shadow of the fiber. (From Chipman et al. 2012)